As a silicon carbide semiconductor device, Japanese Patent Laying-Open No. 7-326755 (Patent Document 1) discloses a trench gate type MOSFET (Metal Oxide Semiconductor Field Effect Transistor) employing a silicon carbide substrate, for example. According to this publication, in order to prevent dielectric breakdown of a gate oxide film at the bottom portion of a trench, the thickness of the gate thermal oxidation film at the bottom portion is made thicker than the thickness thereof at the side portion.